you can play around with the width 235/245/255 and the height of the side wall 65/70/75 but not the last number which is the diameter of the wheel.....sorry dude

go to tirerack.com and click (search by size)> then (all sizes available) you can put 15" in there and it will give all the available size tires you can put on a 15" wheel. click the (specs) link underneath each tire and it will give you a range of wheel widths that each tire can be used on. i havent found another site that allows you to do this!

hint: the middle number is a ratio so if your looking for the biggest tire find the widest one that will fit, then the highest aspect ratio or (middle number). But that doesnt work for flotation tires which are sized by the fully inflated dimensions.